It sounds like one or more of the spring-loaded tabs on the 3-4 synchronizer has slipped out of place, locking it in 4th. gear. It's not an uncommon situation, which usually happens when someone assembles the synchronizer slightly wrong. I hope it won't take a full teardown to fix it, but it's a possibility.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!